Zeolites are crystalline or quasi-crystalline aluminosilicates constructed of repeating SiO4 and AlO4 tetrahedral units. These units are linked together to form frameworks having regular intra-crystalline cavities and channels of molecular dimensions. Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement its tetrahedral units. By convention, each framework type is assigned a unique three-letter code (e.g., “AEI”) by the International Zeolite Association (IZA).
Zeolites have numerous industrial applications, and zeolites of certain frameworks, such as AEI, are known to be effective catalyst for treating combustion exhaust gas in industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas may be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst.
Synthetic AEI zeolites are produced using a structure directing agent (SDA), also referred to as a “template” or “templating agent”. SDAs are typically complex organic molecules which guide or direct the molecular shape and pattern of the zeolite's framework. Generally, the SDA serves as a mold around which the zeolite crystals form. After the crystals are formed, the SDA is removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
In typical synthesis techniques, solid zeolite crystals precipitate from a reaction mixture which contains the framework reactants (e.g., a source of silica and a source of alumina), a source of hydroxide ions (e.g., NaOH), and an SDA. Such synthesis techniques usually take several days (depending on factors such as crystallization temperature) to achieve the desired crystallization. When crystallization is complete, the solid precipitate containing the zeolite crystals is filtered off and the remaining mother liquor is discarded. This discarded mother liquor contains unused SDA, which is often degraded due to harsh reaction conditions.
Known SDAs for use in AEI zeolite synthesis are relatively expensive and contribute to a substantial portion of the cost of manufacturing the zeolite. In addition, conventional methods for synthesizing zeolite AEI have a relatively poor yield on silica (a key component of the reaction mixture) which also impacts manufacturing costs. Accordingly, it would be desirable to reduce the cost of the synthesis process, preferably by a means that has a low impact on the environment. This invention satisfies this need amongst others.